I own both. The UK is definitely underdriven (about half blast), and the M6 is overdriven (to maybe twice or a bit more.
At six volts, the UK pulls 147 mA, the M6 closer to 600. So that's .88 against 3.6 Watts, say four times more power *to the converters*. Hard to say for sure what's actually delivered, but there's no real comparison.
FWIW, I only did a quick check on the M6 but the UK is interesting:
V cur Fc
8 131 1850
7 137 1760
6 147 1700
5 163 1580
4 156 1380
The Fc is only relative, it was maybe 4 inches out and randomly aligned, but shows little real drop off, but does track power input (confirming it). You can see the effect of the regulator as the current drops off as the voltage goes up (or goes up as the battery goes down), it's 'getting into trouble' at four Volts, Vf is very close.
Also note I got brave and took it to 8 Volts. There seems to be little to fear from Li cells being a few tenths high, even a Volt or *two* was OK (in the open, for a few minutes only).
It's also worth noting that the LED is the star version, with the factory heat sink. The reflector body is *glued* on. Overall, a neat thing.
Both are. Both are recommended. But they are really different beasts all together.
Doug Owen